The present invention relates in general to methods and apparatus for fabricating pressure vessels from polymeric components, and more particularly relates to methods and apparatus for joining blow-molded components of such vessels to injection-molded components thereof.
Pressurized vessels made of polymeric materials, such as resin tanks for water treatment systems, are known in the art. According to one conventional method of fabricating such vessels, a body of a tank or tank liner is blow-molded of a thermoplastic material. Once the body is completed, an insert, such as a threaded connector, is attached to the blow-molded body as by screws and, optionally, a sealing ring. Alternatively, such inserts may be glued to the body exterior.
These prior art methods have proven expensive and of doubtful utility; inserts attached by such methods have not produced sufficiently tight seals between the body and the insert, especially if the vessel is to be subjected to internal pressure or temperature cycling during use.
One attempt to overcome these prior art drawbacks is described in Liekens et al., U.S. Pat. No. 4,994,132. According to the Liekens patent, the pressure vessel body is blow-molded from a parison around an insert, the insert being sufficiently spaced from the internal surface of the parison that it is not damaged by heat during sidewall formation. After the blow-molded body has been formed, but before the body has cooled, the insert is moved from a position well inside the cavity toward the inside surface of an opening in the blow-molded body. The insert is then fused with the thermoplastic body material surrounding the opening and the apparatus which had held the insert is withdrawn. The Liekens method has the disadvantages of requiring complex fabrication machinery and requiring the components to be joined to be almost-molten, thus creating the danger of heat deformation and damage to portions of the vessel sidewall remote from the fusion site.
A technique called spin-welding is also known in certain container applications for welding two container components together. U.S. Pat. No. 4,762,249 issued to Fortuna et al. shows a technique for spin-welding a thermoplastic container end to a container body formed as a cylinder out of a composite material. The cylinder has a paperboard body and a thermoplastic coating. The cylinder coating is spin-welded to the container end. Containers of this type are useful for the packaging of nonpressurized fluids such as frozen orange juice and oil. The Fortuna ""249 patent shows a vertical interface (that is, one which is aligned with the cylinder axis) between the thermoplastic layer of the composite cylinder and the thermoplastic can end. Other U.S. Patents issued to Fortuna show similar fabrication methods, such as U.S. Pat. Nos. 4,584,037 and 4,534,751. U.S. Pat. Nos. 4,353,761 and 4,411,726 issued to Woerz show a thermoplastic end member with a downwardly depending flange that is spin-welded to an interior thermoplastic coating of an open end of a composite cylinder made of thermoplastic-lined paperboard. The downwardly-depending flange or fin has a slight inward taper.
U.S. Pat. No. 4,466,845, also issued to Fortuna, shows two blow molded container components (a top and a bottom). Vertically-disposed flanges of the top and bottom halves are fitted together in an interference fit and friction-welded together. These containers are disclosed as relatively small and provided for fluids under pressure such as beer and soda.
U.S. Pat. No. 4,606,470 issued to Barlics shows a neck of a container which is adapted to be permanently closed by spin-welding to a container closure. The spin-welding occurs along a vertical interface and fines generated by the spin-welding are caught in a trough provided for the purpose.
None of these prior art references shows spin-welding as a method of fabricating large, plastic pressurized vessels, where the bonding along cylindrical interfaces may not suffice in maintaining integrity of the seal under long-term pressure.
The present invention provides a method and apparatus for manufacturing a vessel, particularly a large vessel, from a body and a separately formed connector or insert. The body is formed, as by blow molding, from a thermoplastic material and has an orifice with an interior margin. The orifice is formed around an axis. An outer surface of the body bounding the orifice is formed as a noncylindrical surface of rotation. An insert or connector, which is previously formed of a preferably thermoplastic material by a method such as injection molding, has a sidewall adapted to be inserted into the orifice and to have a first interface surface adjacent the interior margin of the body orifice. A second interface surface extends as a flange or skirt radially outwardly from a junction with the sidewall, and is formed as a substantial second noncylindrical surface of rotation that is complementary to the outer surface of the body bounding the orifice.
Machinery is provided to rotate at least one of the body and the insert relative to the other and to move them closer together along the axis until the noncylindrical surfaces of rotation come into contact. Responsive to this contact and to the rotational movement of one component with respect to the other, the noncylindrical surfaces of rotation are spin-welded together to form a fluid-impermeable bond between the two components which is strong enough to withstand fluids under pressure and to withstand separation forces due to differential flexing during temperature cycling.
According to another aspect of the invention, a sacrificial weld bead is provided on the outer surface adjacent the vessel body orifice prior to joining the body and the insert together. The viscosity of the thermoplastic material forming the body, at least in the region surrounding the orifice, is selected to be lower than the viscosity of the insert. In this way, the material of the weld bead is used to create a spin-weld of the two components along their interface.
In yet a further aspect of the invention, the initial profile of the body includes a region which is first contacted during the spin-welding process, and which is sacrificed to create the molten weld bead. A shoulder formed on the body acts as a further welding surface which ensures that the insert and body component will not separate during temperature cycling of the completed vessel.
According to yet another aspect of the invention, the outer surface of the blow-molded body adjacent the orifice is formed as a depression with respect to the outer surface of the body farther away from the orifice. In other words, the outer surface of the body is stepped radially inwardly near the orifice. The second interface surface of the insert is received within this depression. The depression acts to channel the melted material of the sacrificial weld bead so as to place the melted material at a beneficial location, and to create a substantially continuous outer surface once the two components are joined together.